Diaphragms are used to transmit a fluid pressure from one area of a sensor to another. It may be that the process area of a sensor is corrosive or otherwise has a detrimental effect on the sensing elements, or that the sensed pressure needs to be scaled for measurement.
Diaphragm assemblies are commonly made by machining a cavity in a block leaving formed convolutions on the cavity wall opposite the entrance. A diaphragm comprising a thin flexible metal sheet with convolutions corresponding to the convolutions of the cavity is welded to the block material around the cavity entrance. On pressurization, the diaphragm is pressed into the cavity, thereby transmitting the exterior pressure to the interior of the cavity. The diaphragm convolutions expand some, giving the diaphragm greater flexible range. If the diaphragm is pressurized further, the diaphragm is pressed against the cavity wall, mating the corresponding convolutions which then resist further extension of the diaphragm. Without the cavity wall as a stop, the diaphragm my be warped, stretched or otherwise permanently altered by over pressurization, thereby changing the response characteristics of the device.
Problems arise in preserving the accuracy of a diaphragm in actual service. Typically, the cavity block is bolted in place with a process gasket compressed against the sealing surface of the cavity block. The different bolts may be tightened to different forces, or the elements bolted to may shift. Similarly, the forces across the cavity block, may change with temperature, static pressure, bolt relaxation or other factors. A force across the cavity block results which may in turn be transferred to the diaphragm. The diaphragm may, like a drum, then be tightened, twisted, or relaxed with respect to its original condition. The result is an altered response.
Manufacturing a traditional diaphragm device is costly. In particular, machining the convolutions in the cavity block is costly. If the diaphragm is miswelded, leaks, fails or is otherwise defective, the cavity block, with its costly convolutions, ports and other features is sacrificed, along with the defective diaphragm.
Process environments can be highly corrosive, and chemical interaction between diaphragm material and process fluid can degrade diaphragm performance or may cause diaphragm failure. A process diaphragm wetted by the process fluid must then withstand the extreme chemical conditions. Unfortunately, no known inexpensive material resistent to all process fluids is available. As a result, a variety of somewhat exotic and generally expensive materials are used in manufacturing diaphragms. Joining the diaphragm securely to housing is difficult when the housing material is not the same as the diaphragm material. As a result, the expensive material chosen for the small or thin diaphragm is repeated in the heavy housing structures. The housings as a result are needlessly expensive.
It is then an objective of the present invention to provide a pressure responsive diaphragm device. It is a further objective to provide a diaphragm device minimally affected by mounting, temperature or other forces placed on the housing of an incorporating device. It is still a further objective to provide a diaphragm device separable, at least during manufacture, from an incorporating device. It is still a further objective to provide a diaphragm device allowing housing structures made of ordinary materials.